A couple of generations ago in the U.S.A. there were some popular SBS's referred to by various names but the general name that stuck was the "Auto Burglar".
28 ga "Auto Burglar"
These nifty little guns were popular in 20 and 28 gauge. Back then automobiles had running boards and doors that didn't lock (or not very well). A highwayman could jump on the running board of a stopped or slow moving car and through an open window threaten the driver with gun or knife. Pulling one of these out from between the seat cushions could give him a serious inferiority complex.
20 ga Ithaca Auto Burglar
I'd tend to go with the 20 gauge for the lower recoil than a 12 gauge and because it's the smallest size for which buckshot is available. But when you're measuring your target distance in less than 20 feet even the 28ga will be like walking into a vege-matic. Using 20ga #2 buckshot with 18 pellets at 15-20ft will penetrate better and ruin their day.
Load it with slugs and you'd have a pretty close approximation to a light-duty
Howdah pistol.
